In electronically-controlled fuel injection systems, actuator controlled valves (e.g. solenoid valves) are used to control the flow of fuel within the injector, and hence, timing, pressure and quantity of fuel injected into the engine cylinders.
For single-valve injection systems, such as Electronic Unit Injectors (EUIs) and Electronic Unit Pumps (EUPs) a single solenoid valve—known as the “Spill Valve”—is used to control the point, or set of conditions, at which fuel pressure within the injector volume begins to increase. If the valve is open, fuel will be allowed to “spill” to low pressure (the fuel tank). Alternatively, if the valve is closed, the mass of fuel within the injector will undergo pressurization due to the advancing cam-driven plunger reducing the injector volume. Injection of fuel into the engine's cylinder occurs once the fuel pressure within the injector becomes greater than the spring pressure that holds the injector needle closed against its seat, resulting in “injector needle lift”. Fuel injection will continue until the Spill Valve re-opens, spilling fuel to low pressure, resulting in the spring forcing the injector needle to return to its closed position. In this situation, the fuel pressure necessary to lift the needle at the start of injection (known as Nozzle Opening Pressure, or NOP) is related to the force within the needle spring (i.e. spring NOP).
In the case of twin-valve injection systems, a secondary solenoid valve is used to regulate the control pressure applied to the back of the injector needle and, hence, NOP can exceed the needle spring pressure (i.e. variable NOP). This solenoid valve is known as the “Needle Control Valve”. It is a “three-way” valve, in that it exposes the port, whose pressure is to be controlled, to either a high control pressure (when de-energized) or a drain pressure (when energized).
Similar actuator controlled valves are used in common rail fuel injection systems too.
This invention refers to the control of both single and twin valve injection systems.
Valve movement is facilitated by means of an actuator that comprises an electromagnetic stator (a series of coil windings wound around a stator core), through which a current is passed to activate an armature. A valve pin is directly attached to the armature, and subsequent movement of the armature/valve assembly is used to control flow of fuel within the injector. The valve pin is held in the open position by a return spring, therefore any electromagnetic force induced by the solenoid coil is working against the spring to close the valve.
The control of the solenoid valve is divided into two general categories, a so called “pull-in” phase and a “hold phase”.
During the pull-in phase, the armature of the solenoid-controlled valve is caused to close by the application of a first current level through the solenoid coil. During the hold phase a second, lower current level is supplied to the solenoid coil to keep the valve closed.
The driving current provided during the pull-in phase is supplied by a capacitor. The capacitor and associated circuitry provide a further voltage supply means (in addition to the battery) and are hereinafter collectively referred to as the “boost circuit”.
The driving current provided during the hold phase is supplied by applying the standard battery voltage across the solenoid coil in order to provide the second current level. A so-called “chopping circuit” controls the application of the battery voltage so that the required drive current supplied to the actuator throughout the injection is between defined upper and lower hold thresholds.
As the battery voltage decreases, the chopping circuit may constantly apply the battery voltage to the solenoid coil during the entire hold phase of injection in order to maintain the driving current to the solenoid between the desired threshold levels.
In order to maintain precise fuelling using fuel injection engines it is required that either the performance of an individual injector is known or the tolerance band of a group of injectors is well known within tight limits. As a consequence this means that factory limits during production must be tight and engine testing must be sensitive enough to pick up the performance of the injector(s).
However, no matter how good the initial set up, there will be a drift in performance over the life of the injector as components bed in or wear out. In order to address the problem of component performance drift the FIE has to have internal control systems to compensate and such control systems need to be able to detect changes in injector performance.
For electromagnetically controlled valves as described above, the control system may detect changes in valve performance through the detection of changes in the current profile of the coil used to drive valve motion.
The current seen on a coil has a characteristic profile due to the induction effect of a decaying magnetic field and a valve moving through that field affects the current profile (this effect is generally termed back EMF). In particular, when the valve reaches the end of its travel, it will stop moving or bounce off of its seat/stop and this change can be detected as a discontinuity, or “fault”, in the current profile.
Since the change in current profile corresponds to the valve meeting its stop and the valve at this point in its actuated state, it follows that what is being detected correlates with the physical events triggered by the actuated valve. Therefore, the change in the characteristic profile of the current provides an effective way to measure the start of injection or pressure rise without reference to external sensors.
A fault detection system that is able to reliably and efficiently detect the changes in the current profile can then relate the change in the current profile to physical events such as the start/end of pressure and start/end of injection (delivery). This gives initial performance benefits as well as allowing the system to self correct if there are changes in valve response. It follows that one of the main disadvantages of the system without fault is that there is no way to control the injector timing to compensate for any changes that occur over the life of the system. It is known that the injector components can undergo two significant changes after installation, namely the bedding in period and wear caused during normal operation. These two conditions mean that the injector performance deviates from the factory set values over its service lifetime.
There is currently no method to track changes in the valve movement characteristics in situ. Presently the only way to compare the valve performance is by removal from the application and testing in a controlled environment with reference to initial factory data (a ‘before and after’ type test).
Existing fault detection relies on sampling either the voltage or current through the coil during a sampling window and then examining the measurements to determine when the valve has stopped moving. This method of fault detection has a number of shortcomings and performance limitations. One of these limitations is that the fault/sampling window actually adds energy to the system (since a voltage is artificially applied so as to drive additional current into the system) and as such is influencing the system performance. More specifically, the extra energy can extend the time the valve is actuated by adding enough energy to effectively re-actuate the valve or lead to erratic valve timing where the force/energy balance is close to sensitive limits.
Fault windows may also have the problem that the window position has an influence on the position of the current discontinuity that is recorded. The closer the fault (“the discontinuity”) is to the end of the fault window, the more energy has entered the coil windings and as such this will tend to retard the natural progress of the valve (partial re-energization). This means the greater the window length before the fault, the greater the magnitude of the imposed error.
As a result of the effects of window position, any detection routine must be able to rapidly and efficiently evaluate the available data and make a fault decision in the shortest possible time. This means that the detection criteria must be mathematically as simple as possible and be paired with a sufficiently powerful CPU to reduce the negative impacts of having the fault window in the wrong position. Ideally, a decision on the fault status should be decided on a shot to shot basis for the best performance benefits.
Due to the operating environment of the injectors, there is typically a degree of electrical noise (typically high frequency RF) present in the engine system. Appropriate sampling methods and hardware acquisition can reduce this noise to a minimum but a successful fault strategy must also incorporate some form of noise filtering or rejection. Existing methods for fault detection that include digital signal processing are either too slow (mathematically intensive) to avoid the error due to window position or they are insufficiently effective at eliminating noise induced errors.
Since the fault window is a deviation from the natural current decay by forced voltage application, there will always be a measured (i.e. non zero) current associated with it. A key difficulty in prior art fault detection systems is discriminating between a valid fault and a non valid event. In other words, the detection routine must be able to distinguish the difference between a natural current decay profile and a profile with the effects of a change in motion by the armature.
The difference between these two profiles can be subtle and traditionally has been difficult to determine mathematically for the wide range of different possible valve motions. This is further complicated by the range of possible coil response profiles that all give slightly different current decay shapes.
It is therefore an object of the present invention to provide a fault detector and an associated method of detecting valve movements that substantially overcomes or mitigates the above mentioned problems.